Embodiments of the invention generally relate to a control system of a rotary wing aircraft, and more particularly, to a gust alleviating control for a coaxial rotary wing aircraft.
Rigid dual rotor helicopters are those with two coaxial, counter-rotating rotors. Each rotor is typically controlled by a respective independent swash plate and can be commanded with both differential and ganged collective and cyclic controls. Differential controls move the swashplates in opposite directions, while ganged controls move the swashplates in unison. Rigid coaxial rotor helicopters must be designed with sufficient rotor spacing to ensure that the rotor blade tips never touch during any maneuver in the flight envelope. As forward flight speed increases, the advancing portion of the rotor produces more lift than the retreating portion because the relative wind speed is higher. This phenomenon is lift offset, the measure of where the center of lift is generated on the two rotors. The use of collective control also changes lift offset with forward airspeed. Lift offset produces opposing roll moments, thus the rotor roll moments acting on each rotor are substantially opposing. These roll moments and lift offset reduce tip clearance between the rotor blades of the coaxial rotors.
High speed operation of a coaxial rotor helicopter in a gusty environment can also adversely affect tip clearance, because tip clearance generally decreases with speed and load factor. The dynamic nature of wind gusts presents difficulties in manually controlled compensation, as wind gusts can vary rapidly. Furthermore, loads induced by heavy wind gusts can also produce a sizable rotor hub moment. One approach to handling increased rotor hub loads is an increase in rotor hub size during system development. However, rotor hub size increases also result in increased rotor hub weight, which is a notable contributor to overall aircraft weight and thus undesirable.